fix: manual-testing-sandbox/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-KERAS-15268069

Author: snyk-bot

manual-testing-sandbox/requirements.txt

@@ -26,7 +26,7 @@ spacy==3.1.2
 
 # Deep Learning
 torch==2.6.0
-keras==2.6.0
+keras==3.13.2
 
 # Image processing
 pillow==8.3.2